Selection method
　　Several low-pass prototype filter is the basis of modern network synthesis method design filter, a variety of low-pass, high-pass, band-pass, band-stop filter are mostly derived from this feature. For this reason, the design of the filter is simplified and the accuracy is improved.
　　Ideal low-pass filter should be able to make all lower than the cut-off frequency of the signal lossless through, and all higher than the cut-off frequency of the signal should be infinitely attenuated, so the amplitude-frequency characteristic curve rectangular, (Brick-wallfilter). Unfortunately, such desirable features are not achievable, and all designs are simply trying to approximate the characteristics of a rectangular filter. Depending on the selected approximation function, a different response can be obtained. Although the approximation function is varied, the "Butterworth response" or "Chebyshev response" is usually used in consideration of the practical use of the circuit.